In a loom of this type, if the weft thread stored on the supply bobbin is withdrawn directly, owing to the high speed of the lances and the inertia of the weft thread, a "balloon" is formed, i.e. a greater or lesser excess length of unwound yarn depending on the density of the yarn.
When unwinding stops, which takes place at precise points in the cycle of the loom, this excess length of weft forms a loop whose two sides may, depending on their degree of twisting, join together and wind one around the other to form a kink. Generally, this kink cannot come undone itself and it will be introduced into the shed when the pulling lance once more entrains the weft thread, thereby causing a fault in the fabric.
A known solution making it possible to eliminate this drawback consists of effecting a preliminary unwinding of the weft and storing the unwound length provisionally on a device comprising a drum. This device unwinds the yarn at virtually constant speed, which reduces the "balloon" effect. The winding on a drum generally makes it possible to eliminate the kinks in the yarn. The major drawback of this system is its very high cost.